I have a WP/axe/stone tank that does -def, I would never call it a "debuff" set even though I can keep quicksand double stacked with no effort. My claws/regen/fire scrapper has melt armor, but I wouldnt call him a debuffer either.
I think the point zombeh is missing is that the team needed debuff, and they wanted a bunch of it.
Whats next, we going to debate if a defender with tough/weave/provoke can be a tank? Having a few of the desired abilities is not the same as being focused on those abilities.